1. Field of the Invention
The invention relates to a lighting system; and more particularly, to a high intensity discharge lighting system powered by a polyphase axial airgap alternator wherein the phases are substantially non-interacting magnetically.
2. Description of the Prior Art
High intensity lighting systems are in widespread use in commercial and industrial applications, such as the illumination of public venues, both indoor and outdoor, including stadiums, arenas, roadways, parking lots, and the like. These systems are used both in permanent, fixed installations and as part of a mobile arrangement employed for nighttime construction and in emergency or disaster conditions, for example.
Many of these systems employ high intensity discharge (HID) lamps. Known classes of HID lamps include low pressure mercury and sodium vapor, high pressure sodium vapor, and metal halide discharge lamps. Typically the lamp has a transparent glass or preferably a quartz envelope filled at least with inert gas and metal or metal halide material. The lamp includes at least two electrodes connected to a source of electric energy, which ordinarily is supplied either from the electric utility grid or by an alternator mechanically powered by a prime mover, such as an internal combustion or gas turbine engine. In some specialized applications, other energy sources, such as batteries, fuel cells, or the like, may be used.
All of these HID lamp classes share a basic principle of operation, in which an electric discharge is created and sustained between electrically energized electrodes. Different ones of these classes have different numbers of electrodes and different electrode configurations. However, generally stated, all of these HID lamps are started by connecting two or more electrodes to an electrical source. Free electrons and ionized gas are produced within the lamp envelope, by processes that may include one or more of thermionic or cold cathode emission of electrons, or direct dielectric breakdown of the gas. Once a sufficient density of charged species is present, the electric field between the electrodes causes a current flow to be established. Atomic collision processes excite the electrons in some of the atoms in the lamp into non-equilibrium energy states, which subsequently decay with the emission of light, which may include visible and ultraviolet wavelengths. Depending on the type of lamp and the operating conditions, the light may be predominantly in one or more discrete spectral lines or may span a continuum of wavelengths.
Most commonly, HID lamps are started by imposing a starting voltage sufficient to cause an electric arc to be struck across electrodes by dielectric breakdown. After current initially begins to flow, the lamp enters an operating regime that exhibits a negative resistance portion. That is to say, the lamp has a current-voltage characteristic that includes an operating region in which an increase in current results in a decrease in voltage drop across the lamp. By way of contrast, an ordinary conductor solely exhibits positive resistance, so that an increase in current corresponds inexorably to an increase in voltage drop. It is believed that the negative resistance phenomenon is a consequence of increased conductivity that results from an increase in the density of electrons and ions in the gas plasma. As a consequence, an HID lamp circuit is virtually never designed with a constant voltage supply. A potential sufficient to initiate the arc and start the lamp would be highly likely to result in a runaway behavior, in which the lamp would start, then experience an excessive increase in current that would markedly shorten lamp life. Instead, the lamp is ordinarily connected to an AC voltage source through a ballast providing significant inductive impedance. For example, a metal halide HID lamp of the type commonly used in a mobile light tower has a rated steady state output of about 1 kW. The lamp has a recommended starting potential of about 450 V, but a steady-state operation at about 240 V. Although a ballast having but a simple inductor suffices for some HID lamps, more often a combination of inductors and capacitors is used.
A further function of the ballast is to sustain the arc through the zero voltage crossing of the AC supply voltage. Near the zero crossing, the electric field in the lamp is insufficient to sustain the discharge. Thermalization and recombination processes in the plasma result in a decay with time of the number of charged species available for conduction. If the conductivity drops too much while the voltage is near zero, the lamp is extinguished and the arc must be re-established. Excessive cycling is known to decrease lamp life. Two approaches have been proposed to prevent extinguishment. If the ballast has sufficient inductance, the phase shift between the current and voltage and the inherent non-linearity of the lamp are together sufficient to increase the slope of the voltage waveform at the zero crossing. The time spent below the threshold is thus too short for the plasma to decay enough to extinguish the discharge. In other cases, a higher supply frequency is used as a means to increase voltage slope at the zero crossing.
However, both these approaches have undesirable consequences. A ballast with sufficient inductance to function at typical line frequencies of 50-60 Hz is massive and expensive. Ballasts also produce significant core losses, especially if constructed with conventional soft magnetic materials. The detrimental effect of core losses on overall device efficiency is particularly significant in devices operating above line frequency, requiring specific measures to ensure that the substantial waste heat is properly dissipated.
One common application of HID lamp systems is in a mobile light tower, such as that disclosed by U.S. Pat. No. 5,808,450 (hereinafter “the 450 patent”), which is incorporated herein in the entirety by reference thereto. The '450 patent provides a mobile light tower that includes a frame structure, a lighting assembly mounted on the frame structure, and a source of AC electric power. Commonly the tower system is configured as a wheeled trailer that can be towed by a vehicle to a desired location. The lighting assembly comprises a plurality of HID lamps, often four lamps mounted on a retractable, telescoping boom. In its closed position, the boom is relatively compact, permitting the system to be towed conveniently. During operation, the boom is vertically extended and erected, permitting the lamps to illuminate a relatively wide area. A suitable electric power source for the '450 system is said to be a diesel engine driving a synchronous alternator. Other ancillary equipment, including a fuel tank, a starter battery for the diesel engine, and electrical controls are included in the system. Such a system has a number of uses, notably including the illumination of a nighttime construction site. One embodiment of a mobile light tower provided by the '450 patent is depicted by FIG. 1. Light tower 110 has a mobile frame structure 112, such as a trailer having wheels 114 and a hitch 116. Tower 110 has a lighting assembly 118 mounted on the frame structure 112. The lighting assembly 118 has a retractable, telescoping boom. The lower end of the boom 120 is pivotally mounted to the frame structure 112 which a locking hinge 122. A set of four lamps 124, preferably metal halide lamps, are mounted to the far end of the retractable telescoping boom 120 opposite the hinge 122. A source of electric AC power, comprising a prime mover driving a synchronous alternator assembly, is mounted on the mobile frame structure 112 within an alternator assembly housing 126. Electrical AC power provided from the power source energizes lamps 124 through electrical power lines 128. The illustrated mobile light tower 110 has three jacks 130 to support the frame structure 112 in a stationary position. To set up the mobile light tower 110 in preparation of operation of the set of lamps 124, the mobile light tower 110 is towed to a position where it is desirable to set up the light tower 110, and the jacks 130 are engaged. Then, a hand crank 132 is used to pull the boom 120 from a retracted position to an upright position as depicted in FIG. 1. The height of the lamps 124 can be adjusted by adjusting an inner telescoping boom member 134 within an outer telescoping boom member 136. A horizontal light support member 40 is mounted to the top of the inner telescoping boom member 134. The lamps 124 are adjustably attached to a horizontal support member.
However, there are known deficiencies in present mobile light towers, including that provided by the '450 patent. Most common alternators are designed to have a low source impedance and operate at a low frequency, such as 60 Hz. To power HID lamps with a low frequency, low impedance alternator, an intervening ballast must be used to accommodate the highly non-linear electrical characteristics of HID lamps, as discussed hereinabove. The ballast typically must provide at least three functions: (i) increasing the voltage at lamp start-up to a level sufficient to strike the required arc; (ii) limiting current flow during steady-state lamp operation to prevent runaway; and (iii) increasing the slope of the AC voltage waveform through the zero-crossing point of the current waveform to prevent lamp self-extinguishment. Such a ballast adds substantially to the weight, volume, and expense of the system.
Another problem arises with HID light systems wherein a conventional polyphase alternator is connected such that each HID lamp is supplied by one of the alternator phases. Such a multi-lamp system often experiences significant difficulties during the initial startup, attributable to detrimental magnetic interactions between the alternator phases. Frequently, a first one or more of the lamps starts satisfactorily, but parasitic flux paths cause later-to-fire phases to lack sufficient voltage to strike the initial arc in the respective lamp for some appreciable time. Such interactions markedly impair the flexibility of such systems, which cannot be fully and reliably started without a long wait time or the provision of compensating circuitry that considerably complicates the lighting system.
The '450 patent discloses avoidance of the foregoing interaction problem by providing a power source comprising a separate alternator to power each HID lamp. Furthermore, each alternator in the alternative system provided by the '450 patent is said to have internal impedance characteristics that permit it to drive an HID lamp without any external ballasts. The use of separate alternator units disposed on a common shaft obviates the interaction problem, but at the cost of a lighting system that is larger and more expensive and complicated to construct and operate. The '450 alternator system also is said to operate at a relatively high frequency, such as 200-600 Hz, to minimize the risk of self-extinguishment. However, dynamoelectric machines employing conventional soft magnetic material in their stators and operated at commutating frequencies above line frequency are known to experience significant core losses. As a result, they frequently must be equipped with substantial cooling means or be designed to operate at a lower working flux density. These features either reduce efficiency or increase overall device size and weight.
Rotating machines, including the present alternator, ordinarily comprise a stationary component known as a stator and a rotating component known as a rotor. Adjacent faces of the rotor and stator are separated by a small airgap traversed by magnetic flux linking the rotor and stator. It will be understood by those skilled in the art that a rotating machine may comprise plural, mechanically connected rotors and plural stators. Virtually all rotating machines are conventionally classifiable as being either radial or axial airgap types. A radial airgap type is one in which the rotor and stator are separated radially and the traversing magnetic flux is directed predominantly perpendicular to the axis of rotation of the rotor. In an axial airgap device, the rotor and stator are axially separated and the flux traversal is predominantly parallel to the rotational axis.
Except for certain specialized types, motors and generators generally employ soft magnetic materials of one or more types. By “soft magnetic material” is meant one that is easily and efficiently magnetized and demagnetized. The energy that is inevitably dissipated in a magnetic material during each magnetization cycle is termed hysteresis loss or core loss. The magnitude of hysteresis loss is a function both of the excitation amplitude and frequency. A soft magnetic material further exhibits high permeability and low magnetic coercivity. Motors and generators also include a source of magnetomotive force, which can be provided either by one or more permanent magnets or by additional soft magnetic material encircled by current-carrying windings. By “permanent magnet material,” also called “hard magnetic material,” is meant a magnetic material that has a high magnetic coercivity and strongly retains its magnetization and resists being demagnetized. Depending on the type of machine, the permanent and soft magnetic materials may be disposed either on the rotor or stator.
By far, the preponderance of dynamoelectric machines currently produced use as soft magnetic material various grades of electrical or motor steels, which are alloys of Fe with one or more alloying elements, especially including Si, P, C, and Al. Most commonly, Si is a predominant alloying element. While it is generally believed that motors and generators having rotors constructed with advanced permanent magnet material and stators having cores made with advanced, low-loss soft materials, such as amorphous metal, have the potential to provide substantially higher efficiencies and power densities compared to conventional radial airgap motors and generators, there has been little success in building such machines of either axial or radial airgap type. Previous attempts at incorporating amorphous material into conventional radial or axial airgap machines have been largely unsuccessful commercially. Early designs mainly involved substituting the stator and/or rotor with coils or circular laminations of amorphous metal, typically cut with teeth through the internal or external surface. Amorphous metal has unique magnetic and mechanical properties that make it difficult or impossible to directly substitute for ordinary steels in conventionally designed machines.
High speed electric machines are almost always manufactured with low pole counts, lest the magnetic materials in electric machines operating at higher frequencies experience excessive core losses that contribute to inefficient machine design. This is mainly due to the fact that the soft material used in the vast majority of present machines is a silicon-iron alloy (Si—Fe). It is well known that losses resulting from changing a magnetic field at frequencies greater than about 400 Hz in conventional Si—Fe-based materials causes the material to heat undesirably, oftentimes to a point where the device cannot be cooled by any acceptable means.
Accordingly, there remains a need in the art for lighting systems that are highly compact, efficient and reliable. Especially desired are self-contained systems employing alternators that take full advantage of the specific characteristics associated with low-loss material, thus eliminating many of the disadvantages associated with conventional machines. Ideally, an improved mobile lighting system would provide higher efficiency of conversion between mechanical and electrical energy forms and operate for an extended period on a minimal fuel charge. Improved efficiency in generating machines powered by fossil fuels would concomitantly reduce air pollution.